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Pacemakers Pacemakers Presentation Transcript

  • ENTC 4350 Pacemakers
    • A pacemaker is a prosthetic device for the heart, first conceived in 1932 by Albert S. Hymen, an American cardiologist.
      • In 1952 the pacemaker was used clinically by Paul M. Zoll as an external device.
    • With the advent of solid-state circuitry in the early 1960s, it was made into a battery-operated prosthesis that was implantable into the patient.
      • Credit for the implantable pacemaker is given to the American physicians William Chardack and Andrew Gage and to the engineer Wilson Greatbatch.
    • Other heart prostheses, or spare parts, include coronary bypass vessels and artificial heart valves.
      • An especially innovative recent heart prosthesis is the artificial heart, the best known example of which is the Jarvik-7, designed by Robert K. Jarvik and implanted into Barney Clark by William DeVries in 1982.
        • Another implantable artificial heart was developed in 1985 at Penn State University by a team headed by William Pierce.
    • A pacemaker is a prosthesis specifically for the sinoatrial (SA) node of the heart.
      • The SA node may become ineffective for several reasons, among them:
        • the SA node tissue or atrium may become diseased; or
        • the path of the heart depolarization—specifically, the atrioventricular (AV) node from the atrium to the ventricles—may become diseased, producing a heart block.
    • Furthermore, bradycardia, a slowing of the heart rate generally to below 50 or 60 beats per minute (bpm), may develop because of aging or other reasons.
      • These diseases may be treated either with a pacemaker or with medicine, depending upon the case.
    • In the case that the SA node fails to pace the heart properly, the ventricles may beat at their own self-paced rate, normally about 40 bpm.
      • At this heart rate, a patient may survive, but may not be able to function normally.
    • Because the pacemaker is battery-operated and surgically implanted, battery lifetime is one of the most important considerations.
      • The lifetime is determined primarily by the stimulus requirements, as well as the current caused by the pacemaker circuitry.
    • The use of complementary metal-oxide semiconductor (CMOS) integrated circuits has dramatically reduced the current drain, but the stimulus requirements are determined by physiology and cannot be reduced effectively.
    • As is usually the case with physiological stimuli, there is a curve of stimulus intensity versus duration associated with the physiological response of heart depolarization.
    • The figure shows the stimulus voltage, V s , at the tissue-electrode interface.
    V s (V) Time, T D (ms)
    • It has a stimulus duration T D , measured in milliseconds.
      • Such curves depend upon the electrode-heart resistance, R H , which may range from 100 to 1400  .
    • The value of R H may change over time because of tissue scarring at the electrode-tissue interface.
      • In order to produce a stimulus pulse, it is necessary to deliver energy to the electrode with a pacemaker circuit.
    • A pacemaker in its simplest configuration is essentially a battery-operated digital pulse generator.
    • A digital pulse has a voltage V s that may be made variable to allow adjustments in the energy, E P , delivered by the pacemaker to the heart during each pulse.
    • During the pulse duration, T D , the stimulus voltage drives energy into the heart.
      • When the pulse is OFF, it causes an energy drain given by V s I D T , where T is the time period between successive pulses, and I D is the current drain on the battery when the pulse is OFF.
    • Therefore, the energy delivered by the pacemaker during each pulse is given as
    • Using the figure, compute the energy per pulse when the pacemaker pulse width is 0.5 ms, the circuit-current drain is 1  A, the heart-electrode resistance is 200  , and the heart rate is 70 bpm.
    • From the figure, V s = 1.8 V. Also, T = (60/70) s. Then,
      • Thus the energy used for each pulse is
      • EP = 9.643  J/pulse
  • Pacemaker Batteries
    • Battery-operated equipment is convenient in many applications other than pacemakers because it can be used without a power cord, and it is safer because leakage currents are not usually present.
      • The disadvantage is that batteries are relatively large and of limited energy-storage capability.
        • Even so, the energy demand of the pacemaker is such that batteries with lifetimes between five and ten years are available.
    • Mercury cells with two-year lifetimes, used in pacemakers in the past, have been made obsolete by lithium iodide cells which can last as long as 15 years before they need to be replaced.
      • Nuclear pacemaker batteries have been used to extend battery lifetimes to over 20 years, even for dual-chamber pacemakers that use higher amounts of battery power.
    • Nuclear batteries pose an environmental hazard, however, because in an accident the radioactive material could be released into the environment.
      • Nuclear batteries are being considered for artificial implanted hearts also, because of the potential for high energy storage, but this research is only beginning.
    • Rechargeable batteries are not widely used for low power pacemaker application, since their shelf life is no longer than that of a lithium iodide pacemaker battery in normal use.
    • The lifetime of a storage battery depends on both its ampere-hour (A-H) rating and its shelf life.
      • Shelf life is limited self-discharge of the battery due to internal leakage currents, particle migration, formation of insulating layers, and internal shorts.
    • An illustrative example of a battery A-H rating versus its current drain is given in the figure.
    • At high current drain, polarization of the metal electrolyte boundary increases the internal resistance of the battery and decreases the A-H rating.
    • Implantable batteries are usually encased in metal.
      • If they become too hot, such as when shorted, the case may rupture.
        • Pacemaker design should ensure that the case is strong enough to contain such a rupture and prevent toxic materials from entering the body of the patient.
  • Illustrative Pacemaker Characteristics
    • The pacemaker consists of three major components:
      • the lead wire,
      • the electronic pulsing circuit, and
      • the battery.
    • The lead can cause a failure due to metal fatigue, introduced by the motion and beating of the heart.
      • To avoid such fatigue, the lead may be constructed by winding platinum ribbon around polyester yarn.
        • Each lead may have three such wires for redundancy.
    • The pacemaker electrode must make a secure contact with t he heart for several years.
    • To ensure this, two methods of implantation are used under the following classifications:
      • (1) endocardial lead, in which the pacemaker lead is inserted through a major vein through a catheter guide into the right ventricle of the heart; and
      • (2) epicardial lead, in which the pacemaker electrode is sutured to the external wall of the heart during open-heart surgery , and a wire electrode is thereby secured into the tissue.
    • For endocardial lead implantation the electrode may be attached with tines.
    • The tines are pushed into the Purkinje muscle fibers of the ventricle and latch themselves in place.
      • The porous electrode tip minimizes motion between the tip and the tissue so as to reduce the scar tissue buildup.
        • This tends to keep the contact resistance low.
    • The electrode may also be held in place with a helical wire that is screwed into the tissues with a twisting motion.
    • In this case a bipolar electrode a few centimeters behind the contact electrode serves as a return path for current to the pacemaker.
    • In the unipolar pacemaker lead, the second electrode is eliminated, and the return conductive path to the pacemaker is made through body fluids.
      • A unipolar lead electrode may also be held in place by either sutures, tines, or a helical wire.
    • The electrode-muscle contact can change after a time because of
      • (1) polarization by ionic current flow;
      • (2) tissue and scar growth; or
      • (3) mechanical motion of the heart.
    • A symptom of such change may be an increased electrode impedance.
      • The problem may be fixed by increasing the pulse voltage from the pacemaker or by lengthening its duration.
        • Loss of contact altogether may require surgical reimplantation.
    • The implantable pacemaker is presented as a battery-powered, digital pulse generator, and it may be considered an asynchronous type of unit.
      • Other types of pacemaker include the R-wave synchronous, R-wave inhibited, and P-wave synchronous pacemakers.
    • The asynchronous pacemaker produces a pulse at a preset rate, for example 70 bpm, and delivers pulses to the heart regardless of the heart’s natural beating tendency and independent of the QRS complex.
      • This pacemaker does not increase the heart rate in response to the body demand for more blood during exertion.
    • However, a P-wave synchronous pacemaker does.
      • The SA node depolarization responds to body demands through the vagus nerve and hormones transported in the blood.
    • In a P-wave synchronous pacemaker, the SA node triggers the pacer, which in turn drives the ventricle.
      • It is used when the AV node is blocked because of disease.
    • As shown, this pacemaker requires two leads.
      • The atrial lead feeds the atrial pulse back to a sensing amplifier.
      • The driver, connected to the ventricle, delivers the pacing pulse.
    • The R-wave inhibited pacemaker also allows the heart to pace at its normal rhythm when it is able to.
      • However, if the R-wave is missing for a preset period of time, the pacer will supply a stimulus.
    • Therefore, if the pulse rate falls below a predetermined minimum, the pacemaker will turn on and provide the heart a stimulus.
      • For this reason it is called a demand pacemaker.
    • Another type of demand pacemaker uses a piezoelectric sensor shielded inside the pacemaker casing.
      • When this sensor is slightly stressed or bent by the patient’s body activity, the pacemaker will automatically increase or decrease its rate.
    • According to Medtronic, Inc., their model will react to a movement of one-millionth of an inch.
      • It will change heart rates to as high as 150 bpm during vigorous activity or as low as 60 bpm during rest periods.
    • A programmable pacemaker is one that can be altered both in its block diagram and in the size and rate of the pulse it delivers.
    • A pacemaker that can be reconfigured into four different block diagrams, after having been implanted.
    • A magnet may be placed over the pacemaker on the skin of the patient in order to activate a reed switch, which switches the pacemaker into one of the four configurations shown.
      • Another kind of programming is done to alter the delivered stimulus and the pacemaker sensitivity to feedback signals.
    • A programmable pacemaker is shown in the figure.
    • The telemetric programmer may be placed over the pacemaker to select pulse rates ranging from 30 to 155 bpm , feedback sensitivities from 0.7 to 4.5 V, pulse amplitudes from 2.5 to 10 V, and pulse widths from 0.25 to 1 ms, among other parameters.
    • A hard copy of the programming record is provided by the printer shown.
    • When temporary heart pacing is needed, an external pacemaker may be used.
      • Since this device is not implanted, there is no need for extensive surgery.
    • A temporary pacing lead uses a balloon tip, so that the flow of blood will carry the pacing electrode into the heart when the balloon is inflated.
  • The ECG Pattern and Cardiac Pacing
    • The figure shows the appearance of the normal ECG signal as measured in the atrium.
    • Notice the large P wave, which is almost as high as the normal QRS complex.
    • In contrast, this figure shows the effect of adding a continuously operating pacemaker signal to the normal atrium.
    • Now the heart is responding only to the pacemaker, and the pacemaker is said to have “captured” the heart rate.
      • Note that the QRS wave follows the pacemaker-generated P wave at a fixed interval, and that there are no beats generated sinoatrial (SA) node.
    • The pacemaker signal is large enough and occurs at a high enough rate to keep the SA node in the depolarized condition so that it cannot fire.
      • This is important, because an occasional, ectopic, SA-node beat would be entirely out of synchronization with the regularly occurring pacemaker beat.
    • Eventually, a pacemaker-induced pulse would occur during the latter part of the QRS complex or during the T wave from the ectopic SA-node beat, and this would be trouble.
      • It turns out that disease-weakened hearts are more sensitive than healthy hearts to any signal that arrive during the latter part of the QRS complex or the T wave, and such a weakened heart will go into fibrillation if a pacemaker beat and either of these signals happen to coincide.
    • To avoid this hazard, the pacemaker signal is set large enough to preclude the occurrence of any inadvertent SA-node beats.
    • The above mode of pacemaker operation was always used when pacemakers were first invented, and it is still used if the P or QRS waves are weak, very irregular, or entirely absent.
      • This mode has its problems in that no adjustment can be made for the normal change in heart rate from resting to exercise.
    • Usually a rate of 70 heats per minute is used as a compromise.
      • The requirement for continuous operation is reflected in reduced battery life, and the pacemaker has to be changed more often.
    • If a patient has a more nearly normal heart, there may be no need for continuous pacing, and the unit is set in the “demand” mode.
      • In this mode, the pacemaker detects the peak of the QRS wave and begins “counting.”
    • If the next QRS wave occurs within what is called the “capture interval,” the pacemaker does nothing.
      • If the QRS wave is late or absent, the pacemaker stimulates the heart.
        • Here again, the locus of can be in the atrium or in the right ventricle, as necessary.
    • If the QRS wave stops entirely, the pacemaker will stimulate the heart at about 70 beats per minute;
      • One might say that it switches from the “demand” mode to the “continuous” mode.
    • Demand operation results in longer battery life and allows the patient to benefit from the normal heart-rate control system that adjusts the beat to the demands of the body.
      • The pacemaker is available for action if and when the patient’s own heart-rate control system should fail.